1. Field of the Invention
The present invention relates to an image pickup element used for photographing an object.
2. Related Background Art
FIG. 27 is a center cross-sectional view showing the structure of a general image pickup element that forms a color image, i.e., pixels constituting the image pickup element. A micro lens 101 condenses external light beams to increase light acquisition efficiency. Color filters 102 are used to separate the obtained light beams in accordance with their wavelengths, a filter for one of the primary colors, R (red), G (green) and B (blue), is provided for each pixel. In addition, complementary color filters using C (cyan), M (magenta) and Y (yellow) are also used as color filters. Conventionally, an image pickup technique has been widely employed whereby these pixels are arranged like a mosaic and luminance data and color data corresponding to the number of pixels are prepared in the succeeding signal process. The color filter array conventionally used for an image pickup element tends to be a Bayer array.
In FIG. 27, further provided are a silicon wafer 104; a photoelectric conversion section 103, for converting received light into charges; a Poly wiring layer 110, serving as a gate for controlling the charges produced by the photoelectric conversion section 103; and wiring layers 111 to 113, made of a metal such as aluminum.
However, this conventional example has the following problems. Generally, the photographic processing employed to obtain a preferable image characteristic includes a first process of forming an image of an object using an optical device; a second process of adjusting the object image to reduce a high frequency component in a spatial frequency property; a third process of photoelectrically converting into an electric signal the object image for which the spatial frequency property has been adjusted; and a fourth process of employing the spatial frequency to compensate for a response to the obtained electric signal. At this time, the optical image is sampled by an image pickup element having a definite number of pixels. In order to output a high-quality image, in accordance with the sampling theory, a frequency component equal to or higher than the Nyquist frequency inherent to the image pickup element must be reduced in the spatial frequency property of the optical image. The Nyquist frequency is ½ of the sampling frequency determined by the pixel pitches. Therefore, through the optimized process sequence, the optical image to be sampled is adjusted to obtain an optical image having a characteristic corresponding to the Nyquist frequency inherent to the image pickup element, so that a high-quality image can be obtained wherein aliasing distortion, i.e., moire, is not remarkable.
A modulation transfer function (MTF), which is the spatial frequency transmission characteristic of an image, is an evaluation value that can well represent the characteristic of the sharpness of a digital still camera or a video camera. Specific elements affecting the MTF are an image forming optical system which is an optical device, an optical low-pass filter which is used to limit the bandwidth of an object image, the shape of an opening formed in the photoelectric conversion area of the image pickup element, and a digital iris correction function. An MTF that represents a final, overall image characteristic is provided as a product of the MTFs of the individual elements. That is, only the MTFs for the first to the fourth processes must be obtained to calculate the product.
It should be noted, that since the digital filter process, which is the fourth process, is performed on an image that has already been sampled by the image pickup element, no consideration need be given for a high frequency that exceeds the Nyquist frequency.
Therefore, according to the configuration, wherein a frequency component equal to or higher than the Nyquist frequency, inherent to the image pickup element, is reduced in the spatial frequency characteristic of the optical image, the frequency component at the Nyquist frequency or higher is small in the MTF product obtained for the first process, for the second process and for the third process, but not for the fourth process. On the assumption that a still image will be viewed, as is the case for a digital still camera, consideration should be given to the fact that, when a response value at a frequency slightly lower than the Nyquist frequency is high, even though the frequency component higher than the Nyquist frequency remains small, an image having a higher resolution is formed more easily than when there is no high frequency component higher than the Nyquist frequency.
In the first process for forming an object image using the image forming optical system, generally, an optical aberration in the center of the screen is more easily corrected than is one on the periphery. In order to obtain a preferable image in the peripheral portion of the screen, an especially acceptable characteristic close to the diffraction limit MTF which is determined using the F number of an photographic lens, must be obtained for the center at the screen. This need has recently been magnified as the pixel size of image pickup elements has continued to be reduced. Therefore, for the image forming optical system, it should be assumed that the image forming optical system is an ideal lens having no optical aberration to consider an MTF.
Further, in an image pickup element where light-receiving openings having a width d are laid out with no intervening gap, since the widths of the light-receiving openings match the pixel pitches, the response value of the third process at the Nyquist frequency u=d/2 is quite high. For this reason, generally, the frequency component around the Nyquist frequency is trapped in the second process in order to reduce the total MTF around the Nyquist frequency.
In the second process, normally, an optical low-pass filter made of a material, such as rock crystal, having a birefringence characteristic is employed. Or instead, a diffraction grating of a phase type, described in Japanese Patent Application Laid-Open No. 2000-066141, may be employed.
A birefringence plate is so interposed in the optical path of the optical device that the optical axis of the plate is inclined parallel to the horizontal direction of the image-forming face, and an object image produced by an ordinary ray and an object image produced by an extraordinary ray are formed while being shifted horizontally and relative to each other by at a predetermined distance. The birefringence plate is used to trap a specified spatial frequency, since when the object images are shifted, a bright band and a dark band on the fringes of the spatial frequency are overlapped. The MTF that uses the optical low-pass filter is represented by the following equation (1):R2(u)=|cos(π·u·ω)  (1).
In this equation (1), R2(u) denotes a response, u denotes a spatial frequency for an optical image, and ω denotes an object image separation width.
When a birefringence plate having an appropriate thickness is selected, a response of zero can be obtained for the image pickup element at the Nyquist frequency. And when the diffraction grating is employed, an optical image need only be separated into a plurality of images by diffraction, and these images are overlapped at predetermined locations to obtain the same effects.
However, in order to fabricate a birefringence plate, a crystal such as rock crystal or lithium niobate must be grown and polished to reduce its thickness, and this greatly increases the processing costs. Further, since a very fine structure is required for the diffraction grating, this also increases the processing costs.
The usage efficiency of light will now be described. For example, for a CCD image pickup element, for which the pixels of primary color filters that are intended to provide high-quality color reproduction are arranged in a mosaic shape, R (red), G (green) and B (blue) optical filters are positioned, one by one, between the micro lens 2 and the photoelectric conversion area 3.
At this time, at a pixel for which a red optical filter is provided, only a red light is photoelectrically converted, and a blue light and a green light are absorbed by the optical filters and only produce heat. Similarly, for a pixel for which a green optical filter is provided, a blue light and a red light are not photoelectrically converted and only produce heat, and for a pixel for which a blue optical filter is provided, a green light and a red light are not photoelectrically converted and only produce heat. That is, for the individual pixels of a conventional color image pickup element, of the incident light flux, only light that can pass through a predetermined optical filter is photoelectrically converted and output as an electric signal, while light that can not pass through the predetermined optical filter and only produces heat is discarded.
FIG. 28 is a graph showing a spectral transmittance characteristic for the RGB color filters provided for the image pickup element. Actually, since for an infrared ray the transmittance is high, an infrared cut filter, for blocking wavelengths of 650 nm or higher, is additionally provided between the image pickup element and the pickup lens. And as is apparent from the graph, only about ⅓ of the visible radiation produced by a pixel is effectively employed.
The usage efficiency will now be described in more detail for the individual RGB colors. The dimension ratio for the RGB pixels of the color image pickup element in the Bayer array in FIG. 28 is ¼: 2/4:¼, when the dimension of a unit constituting a regular array is defined as 1. Therefore, the rate for a green light, used when the total light quantity is defined as 1, is ⅓× 2/4=⅙, which is a product of the term for the wavelength selection and the term for the dimension ratio. The usage rate for the red light and for blue light is ⅓×¼= 1/12. Since the total of the usage rates for three lights is ⅙+ 1/12+ 1/12=⅓, in this case, as well as in the above case, the usage efficiency is ⅓. When the total light quantity is defined as 1, ⅔× 2/4=⅓ of the green light and ⅔×¼=⅙ of the red light and of the blue light are not effectively employed.
An image pickup element that uses primary color filters has been employed; however, for an image pickup element that uses complementary color filters, about ⅓ of the visible radiation is not photoelectrically converted, and is not effectively employed. As is described above, for the conventional single-chip image pickup element that uses either primary color filters or complementary color filters, the light usage efficiency is low because the image pickup plane is divided by the color filters.